Previously, various adhesives mainly composed of polymer emulsions have been proposed.
In Japanese Patent Publication No. 51-30576, there has been proposed "a water-resistant adhesive comprising an aqueous emulsion which contains polyvinyl alcohol and in which an isocyanate compound or an isocyanate polymer (hereinafter sometimes referred to as an isocyanate), and an extender and/or a filler reactive to said isocyanate are compounded".
However, when the isocyanate is thus directly compounded in the aqueous polymer emulsion containing water, the reaction proceeds with violence by the reaction of the isocyanate with water, which causes severe foaming and a significant increase in viscosity of the adhesive, a shorter available time, deteriorated coating workability and a remarkable reduction in adhesive strength. These cause serious obstacles on practical use.
Then, in order to remove such disadvantages, it has come to be carried out that the isocyanate once dissolved in a solvent is compounded in the aqueous polymer emulsion.
Namely, in Japanese Patent Publication No. 51-30577, there is proposed "a water-resistant adhesive comprising an aqueous emulsion which contains polyvinyl alcohol and in which an isocyanate compound or an isocyanate polymer dissolved in a solvent having little mutual solubility with water, but having mutual solubility with said isocyanate is dispersed". Further, in Japanese Patent Publication No. 58-29826, there is proposed "a water-resistant adhesive comprising an aqueous emulsion in which (1) polyvinyl alcohol, (2) a compound having an isocyanate group dissolved in a hydrophobic solvent and (3) a surface active agent and/or a formalin condensation resin are compounded".
As described above, these adhesives intend to prolong the available time by the use of the solvent.
However, there occurs the phenomenon that also these adhesives foam, increase in viscosity and thereby rapidly decrease in water resistance after an elapse of about 1 to 2 hours, when used at a temperature of 30.degree. to 40.degree. C. in summer. This brings about the situation that the temperature of a working room must be reduced to 25.degree. C. or lower, or that the adhesives must be used up within a short period of time such as 1 to 2 hours.
On the other hand, there is advanced an emulsion adhesive using acetoacetylated polyvinyl alcohol as a protective colloid.
The adhesive of a vinyl acetate polymer emulsion using the acetoacetylated polyvinyl alcohol as the protective colloid is superior in water resistance, when compared to the adhesive of the vinyl acetate polymer emulsion using the polyvinyl alcohol not acetoacetylated as the protective colloid as described above. This adhesive is therefore used as an adhesive for paper, furniture and thin veneer. However, in the water resistance of the adhesive concerned, the adhesive property is apt to be insufficient for fields in which the high water resistance is required, including the above-mentioned applications, for example, for applications such as laminated lumber, high-grade furniture, thick veneer and flash boards, and therefore a problem is practically encountered.
Under present conditions, it is intended to improve the water resistance of the emulsions concerned by compounding formalin resins such as urea-formalin resins and melamine-formalin resins in vinyl acetate polymer emulsions as means to solve this problem. However, these methods have the disadvantages that the available time is short and that formalin is released from furniture to produce a pollution problem.
On the other hand, in Japanese Patent Publication No. 59-1581, there is proposed an adhesive composition for wood comprising a vinyl acetate polymer emulsion in which the acetoacetylated polyvinyl alcohol concerned is used as the protective colloid and to which an isocyanate compound, an epoxy compound, a radical-forming compound and an oxidizing agent are added as agents for imparting water resistance (agents for improving water resistance).
However, addition of such agents for imparting water resistance deteriorates the pot life (shelf life), and therefore its improvement is desired from the point of workability. Further, when the isocyanate compound or the like is added, the adhesion is reduced with time.
Furthermore, in Japanese Patent Unexamined Publication No. 48-72235, there is proposed a polyvinyl acetate emulsion adhesive comprising a polyvinyl acetate emulsion which contains a hydrophilic protective colloid and in which zirconyl nitrate and zirconyl chloride are incorporated.
However, adhesive compositions containing metal salts such as zirconyl nitrate and zirconyl chloride have the problems that they show strong acidity in pH to cause pollution and deterioration of wood, and that they are weak in adhesion for fields in which high water resistance is required.
Moreover, in Japanese Patent Unexamined Publication No. 60-24077, there is proposed "an aqueous dispersion two liquid-type adhesive comprising a main adhesive component mainly composed of an aqueous emulsion containing acetoacetylated polyvinyl alcohol and an epoxy compound, and a curing agent component mainly composed of an isocyanate compound.
Namely, this adhesive is one in which there is used the aqueous emulsion containing the epoxy compound in addition to the acetoacetylated polyvinyl alcohol.
However, this adhesive has the problem that the retention time of the adhesive strength (change in adhesive strength with time) is short, though slightly changes in viscosity and is good in apparent condition.
Then, it is an object of the present invention to provide an adhesive composition solving such prior-art disadvantages.
The above and other objects and features of the present invention will become apparent from the whole description of this specification.